1. Field of the Invention
The present invention relates to an MRI apparatus, and particularly relates to an MRI apparatus that can correct variations in an MR-signal frequency caused by temporal variations in a readout gradient magnetic field.
2. Description of the Related Art
A magnetic resonance imaging (MRI) method is an imaging method in which a nuclear spin in a subject tissue placed in a static magnetic field is excited with a high-frequency signal (RF pulse) of a Larmor frequency, and image data is reconstructed from a magnetic resonance signal (MR signs)) generated along with the excitation.
An MRI apparatus is a diagnostic imaging apparatus that creates image data based on MR signals detected from the inside of a living body, and can obtain various diagnostic information, such as biochemical information and functional diagnosis information as well as anatomical diagnosis information. Therefore, the MRI apparatus is indispensable to a field of recent diagnostic imaging.
The MRI apparatus includes a magnetic-field generating unit that generates a static magnetic field and various gradient magnetic fields to a subject, a transmitting/receiving unit that irradiates RF pulses to an imaging target portion of the subject to which the magnetic fields are applied, and performs various signal processing on an MR signal detected from the imaging target portion irradiated with the RF pulses, an image-data creating unit that creates image data by performing reconstruction processing on the processed MR signal, and a display unit that displays thereon the created image data. The transmitting/receiving unit includes a transmitting coil that irradiates RF pulses to the subject, a transmitting unit that activates the transmitting coil, a receiving coil that detects an MR signal generated from the subject, and a receiving unit that performs signal processing, such as frequency conversion and A/D conversion, on the MR signal obtained by the receiving coil.
The receiving unit includes, for example, as shown in FIG. 9, a frequency converter 101, an A/D converter 102, a quadrature detector 103, and a filtering processing unit 104. The frequency converter 101 converts the frequency of an MR signal detected by the receiving coil. The A/D converter 102 performs an A/D conversion on the MR signal converted in frequency. The quadrature detector 103 converts the A/D-converted MR signal into a complex MR signal having an I component and a Q component by performing quadrature detection. The filtering processing unit 104 eliminates a needless component, such as a noise, by performing filtering processing on the MR signal.
An MRI apparatus sets a slice encoding direction, a readout encoding direction, and a phase encoding direction, which are orthogonal to each other, with respect to an imaging target portion, and applies gradient magnetic fields in the respective directions, thereby setting an image slice cross-section and adding positional information to the MR signal generated from the image slice cross-section. The receiving coil detects an MR signal in time series generated from the subject with a readout gradient magnetic field that is applied after the transmitting coil irradiates an RF pulse to the subject and a certain time period passes over.
In such case, because the frequency of an MR signal is proportional to the strength of an applied magnetic field, it is desirable that magnetic field strengths are uniform during a collection period (readout period) of the MR signal in order to collect an MR signal with a constantly stable frequency from a certain position. However, it is difficult in practice to obtain a magnetic field in which activation time is short and strength is substantially uniform during the readout period, such as a magnetic-field characteristic with rectangle or trapezoid shape, due to constraints, such as a limitation of a gradient magnetic field power-source or the like, consequently, a sinusoidal-shaped readout gradient magnetic field is used in many cases.
When applying a readout gradient magnetic field having such magnetic-field characteristic to the subject, the frequency of an MR signal generated from the same portion varies along with temporal variations in the magnetic field strength. As a result, when creating image data by performing the A/D conversion and then the reconstruction processing on the MR signal collected in the readout encoding direction, an image distortion caused by the variations in the frequency of the MR signal arises in the image data obtained by performing a Fourier transform on the MR signal obtained by sampling at an equal interval.
To reduce image distortions arising due to variations in the magnetic field strength, a method is proposed such that a time interval is preset to have a constant time-integral value of a gradient magnetic field in an arbitrary waveform applied to the subject, an MR signal obtained by sampling at an unequal interval based on the time interval is rearranged at an equal interval, and then filtering processing and image reconstruction processing are performed (for example, see Japanese Patent No. 3112926).
According to the method described in Japanese Patent No. 3112926, to begin with, an MR signal is extracted by sampling at an unequal interval set based on temporal variations in a readout gradient magnetic field, and then a needless component mixed in the obtained MR signal is eliminated by performing filtering processing on the MR signal. In other words, because the filtering processing for eliminating a needless signal is performed on the MR signal of which MR-signal frequency is corrected, such processing requires a number of periods, consequently, the method has a problem that a real-time quality of image data is deteriorated.